The present invention relates to a two-way ranging system wherein an interrogator transmits pulse pairs as interrogation signals to a transponder, the transponder, after a fixed delay following the reception of the interrogating pulse pair, transmitting a pulse pair as a reply signal to the interrogator if the received interrogating pulse pairs have a predetermined spacing and the distance between the interrogator and the transponder is derived in the interrogator from the time difference between the transmission of the interrogating pulse pair and the reception of the reply pulse pair, taking into account any built-in equipment delays.
Such a two-way ranging system is described in a book by E. Kramar entitled "Funksysteme fur Ortung und Navigation", Verlag Berliner Union GmbH, Stuttgart, 1973, on pages 147 to 159.
A two-way ranging system which has been introduced worldwide is the ICAO (International Civil Aviation Organization) standard DME (distance-measuring equipment). The operating data for DME, including the maximum bandwidth of the carrier wave of the DME pulse pairs and the shape of the DME pulse pairs, are specified in Annex 10 to "Aeronautical Telecommunications of the ICAO".
Even with an optimum shape of the pulse pairs both in the interrogator and in the transponder, there is a maximum measurement accuracy in practice. For given applications, however, it is desirable to further improve this accuracy.